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Stereophonic Sound

by Dennis Faas on September, 28 2005 at 08:09AM EDT

Stereophonic sound, commonly called stereo, is the reproduction of sound, using two independent audio channels, through a pair of widely separated speaker systems, in such a way as to create a pleasant and natural impression of sound heard from various directions as in natural hearing.

The word "stereophonic", from Greek stereos = "solid" and phone = "sound", was coined by Western Electric, by analogy with the word "stereoscopic". Western Electric first demonstrated it at an SMPTE meeting in 1937, then to the general public at Carnegie Hall in 1940.

In popular usage, stereo usually means 2-channel sound recording and sound reproduction using data for more than one speaker simultaneously.

In technical usage, stereo or stereophony means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded. A stereo system can include any number of channels, such as the multichannel audio 5.1- and 6.1-channel systems used on high-end film and television productions. However, it more commonly means only two-channel systems.

Stereo Recording

Stereo is derived from the term stereographic projection, which here generates a stereo image during playback. During two-channel stereo recording, two microphones are place in strategic locations in relation to the source, both record at once. Each channel will be similar, but each will have distinct time-of-arrival difference and sound pressure level difference information. On playback, the listener's brain uses the subtle differences in timing and level to triangulate the positions of the recorded objects.

Stereo recordings often cannot be played by monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, constructive and destructive interference can occur, if both tracks are played on the same speaker. This phenomenon is known as comb filtering.